1. Field of the Invention
The present invention relates to an optical coupler, and more particularly, to a modal evolution optical coupler in which a low-loss optical transmission is accomplished, and a method for manufacturing the coupler.
2. Description of Related Art
Propagation of light by reflection and refraction is determined by the incident angle of the light and the refractive indices of two media. Accordingly, the development of a medium capable of more effectively propagating light is very important in optical communications. An optical fiber is the most widely used medium for transmitting light. The optical fiber includes an internal core for transmitting light and a cladding having a different refractivity for accomplishing a total reflection of light in the core. A core having a diameter ranging from about 8 .mu.m to 200 .mu.m (0.2 mm) is usually used for transmitting light. The small diameter of the core portion causes significant technical difficulties in connecting optical fibers or branching and coupling transmitted light.
An optical coupler is a passive device for branching or coupling an optical signal. The function of branching or coupling an optical signal in optical communications can be simply performed by various photomechanical connections, similar to branching or coupling in electric communications. However, the optical signal cannot be simply realized because of the characteristics of the optical fiber, so that a special optical coupler is employed as a light branching and coupling device.
An optical coupler using an optical fiber which has been the most widely used since the 1970's is a fused coupler which uses an evanescent field coupling method. The fused coupler is manufactured by twisting several optical fibers together and then commonly fusing and tensing the twisted fibers. Every dielectric single-mode waveguide including an optical fiber has an evanescent electromagnetic field which decreases exponentially to the exterior of a core. Accordingly, when two single-mode waveguides are disposed adjacent to each other, a wavemode is excited by an evanescent field of adjacent cores and a combination of optical signals occurs. Such a combination is called an evanescent field combination, and is used by the fused coupler. However, the fused coupler requires a complicated and time consuming manufacturing process, is expensive and it is not likely that the price of the fused coupler will go down.
As another coupler, a waveguide coupler may be used. The waveguide coupler branches or couples light by a quartz glass waveguide formed on a silicon substrate or a waveguide using an ion exchange glass. A small coupler can be produced in large quantities when the waveguide technique is used. However, several technical problems remain such as losses of the waveguide itself, as losses in an optical fiber connection or an improvement in a waveguide forming technique.
As still another coupler, there is a direct-combination core extension coupler using an extension element which has extended the core of an optical fiber in a channel transmission path. The extending element extends light transmitted via a core to have a much larger section centered on the core. An optical coupler can be designed by applying the extension element. Several optical fibers having well cut ends are put together in order. When the extension element is formed from the cut section of each optical fiber, the section areas of the extension elements increase. Consequently, the extension elements touch and combine with each other at a certain distance or more. Once the extension elements combine with each other and form a body, light is branched or two or more light beams are coupled. The coupler easily connects light beams to each other compared to the fused coupler or the waveguide coupler, thereby allowing simple branching or coupling of optical fibers. Also, there is a high probability that the price of the coupler will go down. However, since the core extension is long when extending a core in the channel transmission path, an extension element formed by ultraviolet rays tends to move. Also, light transmission losses are increased by an influence of a resin existing between the section of an optical fiber and the wall of a channel transmission path, etc. The performance of the coupler is remarkably deteriorated, so that the coupler cannot be effectively used as a coupler.
The following patent each disclose features in common with the present invention but do not teach or suggest the specifically recited modal evolution optical coupler of the present invention: U.S. Pat. No. 5,278,926 to Doussiere, entitled Widened Output Mode Semiconductor Optical Component And Method Of Fabricating It, U.S. Pat. No. 5,123,069 to Okayama et al., entitled Waveguide-Type Optical Switch, U.S. Patent No. 4,291,939 to Giallorenzi et al., entitled Polarization-Independent Optical Switches/Modulators, U.S. Pat. No. 4,181,399 to McMahon et al., entitled Optical Internal Reflectance Switchable Couple,; U.S. Pat. No. 5,537,497 to Nelson, entitled Optimized Electrode Geometries For Digital Optical Switches, U.S. Pat. No. 5,586,205 to Chen et al., entitled Apparatus For Selecting Waveguide Modes In Optical Fiber And The Method Of Manufacturing The Same, U.S. Pat. No. 5,475,777 to Imoto et al., entitled Optical Device With A Pig Tail Optical Fiber And Its Production Method, U.S. Pat. No. 5,479,546 to Dumais et al., entitled Optimized Non-Linear Effect Tapered Optical Fiber Interferometer/Switch Device, U.S. Pat. No. 5,265,178 to Braun et al., entitled Fiber Optic Data Communication System, U.S. Pat. No. 5,301,252 to Yanagawa et al., entitled Mode Field Conversion Fiber Component, U.S. Pat. No. 5,337,380 to Darbon et al., entitled Method Of Limiting Coupling Losses Between Monomode Optical Fibers Using A Piece Of Multimode Optical Fiber, U.S. Pat. No. 5,179,606 to Kaihara et al., entitled Optical Coupler, U.S. Pat. No. 5,675,679 to Yuuki, entitled Light Branching And Coupling Device, U.S. Pat. No. 5,515,464 to Sheem, entitled Optical Fiber Interconnections Using Self-Aligned Core-Extensions, U.S. Pat. No. 5,410,626 to Okuta et al., entitled Optical Coupler Having A Tapered Fused Region, and U.S. Pat. No. 5,420,948 to Byron, entitled Chirped Optical Fiber Filter.